A lean burn internal combustion engine is an internal combustion engine in which an air fuel ratio--the ratio of the quantity of intake air (intake amount) to the quantity of fuel--is larger than a theoretical (stoichiometric) air fuel ratio; that is, it is operated at a lean air fuel ratio wherein the intake amount is larger than the fuel amount. In controlling a conventional lean burn internal combustion engine, for example, as described in Japanese Unexamined Patent Publication No. 34329/1976, a method has been proposed in which fuel consumption is improved by controlling the air fuel ratio in accordance with periodic variation of average pressure in a combustion chamber, and increasing an operating region of lean burn. Further, Japanese Unexamined Patent Publication No. 160530/1983, discloses a method in which exhaust gas recirculation, ignition timing and air fuel ratio are controlled in accordance with a variation in torque, which is calculated by combustion pressure and the like.
However, in increasing the operating region of lean burn to improve fuel cost, the objective of reducing a poisonous component of exhaust gas (emission of a nitrogen oxide component, hereinafter referred to as NOx), is not considered; and therefore, the control of a lean burn internal combustion engine is incomplete. Further, the reduction of poisonous components in exhaust gas and the operational performance deteriorate over time in comparison with their initial states, due to aging of component parts of the lean burn internal combustion engine and component parts of a control device.
In addition, the lean burn air fuel ratio is switched to the theoretical air fuel ratio when large power output is required since the output is insufficient in the lean burn state in comparison with the combustion state at the theoretical air fuel ratio. However, in switching the air fuel ratio, the fuel amount is changed while the intake amount remains constant, and therefore the output torque is abruptly changed, adversely affecting handling of the vehicle.